Research brief: New formula supersedes Priestley-Taylor equation for evaporation

Most people imagine that as temperature rises, evaporation increases. While that is true under specific circumstances, in general this is not the case. This is because when evaporation increases it causes a surface to cool and thereby reducing the temperature.

Currently, practical applications based on the theories/models of wet surface evaporation use the Priestly-Taylor model to get results. This model states that net radiation and surface temperature (or near surface air temperature) are independent inputs that determine the rate of evaporation.

However net radiation and surface temperature
are not independent of each other and neither are they independent of
evaporation. The complex interaction between radiation, evaporation and surface
temperature has been the subject of intense study for 50 years and is of
particular importance to the hydrological, meteorological and agricultural
communities.

Researchers from the Centre of Excellence for
Climate Extremes developed a new theoretical formula that acknowledges the
various interactions that occur. For example, as surface temperature increases
a larger fraction of net radiation goes into the process of evaporation. However,
at the same time, the increase in surface temperature causes the net radiation to
decline because of an increase in outgoing long-wave radiation. The consequence
of this interaction is that the evaporation that occurs must be the maximum
possible value.

To test this new formulation the researchers
used newly available satellite data for radiation to investigate how radiation,
evaporation and temperature interact over the global oceans.

The results showed that at global and local
scales the observed evaporation from oceans occurs at the maximum possible rate
calculated by the new formulation, which means that the evaporation process
provided the maximum possible cooling at the ocean surface.

These results suggest that the concept of maximum evaporation reported here is a natural attribute of an extensive wet evaporating surface. This provides a fundamental new insight as to how radiation, evaporation and temperature are interlinked, which opens the way to use these results to investigate how temperature might evolve into the future.